1. Field of the Invention
The present invention relates to a wavelength displacement correcting system for use in a polychromator, particularly for use in a polychromator provided with plastic molded parts to correct a wavelength displacement of the polychromator with time and/or under heat.
2. Description of the Related Art
Conventionally, there has been popularly used a polychromator, as a spectral device for use in a spectral apparatus such as a spectrocolorimeter or a spectroradiometer. The polychromator is constructed in such a manner that wavelength dispersed beams by diffraction on a diffraction grating are simultaneously received on a light receiving array. Considering production cost reduction, it is desirable to use plastic molded parts as primary elements of the polychromator such as a diffraction grating or a housing. The plastic molded parts, however, unavoidably undergo a change with time and/or under heat after the molding, with the result that a wavelength displacement is unavoidable. There are two kinds of wavelength displacement. One is a displacement or change in dispersion width, primarily resulting from a change in grating constant by thermal expansion of a diffraction grating. The other is a uniform displacement or change primarily resulting from a positional displacement in optical arrangement. The former is a displacement proportional to wavelength, and the latter is a displacement which is not proportional to wavelength. There are known technologies to correct these displacements, for instance, the first conventional art disclosed in Japanese Unexamined Patent Publication No. 2005-69784 (D1) i.e. counterpart U.S. Pat. No. 7,116,417B2, and the second conventional art disclosed in Japanese Unexamined Patent Publication No. 2000-298066 (D2) i.e. counterpart U.S. Pat. No. 6,606,156B1.
D1 discloses use of a monochromatic beam from an LED (light emitting diode) of a low cost and yet with an unstable wavelength output, as a reference beam by measuring the wavelengths of a first-order diffracted beam as a first-order beam, and a second-order diffracted beam as a second-order beam. Specifically, D1 utilizes a phenomenon that a positional displacement in optical arrangement causes an identical wavelength displacement with respect to the first-order beam and the second-order beam, whereas a wavelength displacement of the light emitting diode causes a wavelength displacement with respect to the second-order beam twice as large as a wavelength displacement with respect to the first-order beam. With use of the phenomenon, the wavelength displacement resulting from a positional displacement in optical arrangement is detected and corrected by eliminating the wavelength displacement of the light emitting diode. In D1, however, it is impossible to distinguish a wavelength displacement of the light emitting diode from a change in dispersion width, because in both of the cases, the wavelength displacement with respect to the second-order beam is twice as large as the wavelength displacement with respect to the first-order beam. Accordingly, D1 fails to correct a change in dispersion width resulting from a change in grating constant, which may be serious in the case where a plastic diffraction grating subjected to a large thermal expansion is used.
In D2, a plastic material is used for a diffraction grating and a housing. By using the plastic material for the diffraction grating and the housing, a decrease in dispersion width resulting from a change in grating constant by thermal expansion is cancelled by an increase in dispersion width by thermal expansion of the housing, and a uniform wavelength change resulting from a positional displacement in optical arrangement by thermal expansion is cancelled by a thermal expansion of a support member of a light receiving array made of a material different from the material of the housing. D2, however, is proposed on a premise that the entirety of the polychromator is uniformly subjected to a temperature change. Accordingly, D2 not only fails to cope with a non-uniform temperature change but also fails to cope with a wavelength displacement resulting from a cause other than the temperature change such as a change with time.